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Carbon-13 "magic-angle" sample-spinning nuclear magnetic resonance studies of human myelin, and model membrane
Abstract:
We have obtained high-field (11.7 Tesla), high-resolution carbon-13 solid-state "magic-angle" sample-spinning nuclear magnetic resonance (NMR) spectra of a variety of phospholipids, sphingolipids, myelin and white matter samples, resolving and assigning over 40 resonances in the spectra of human and bovine myelin. The NMR results indicated no large spectral changes due to sample preparation, sample freezing, or brain location, and also no changes in myelin structure detectable via light microscopy, electron microscopy, thin layer chromatography, or sodium dodecyl sulfate-polyacrylamide gel electrophoresis, attributable to the sometimes lengthy NMR data acquisition process. Human myelin and white matter chemical shift assignments were made based on 13C "magic angle" sample spinning (MAS) NMR spectra of individual model lipids, as well as on spectra of lipid mixtures. In all myelin samples there were essentially no features attributable to membrane proteins, with the exception of one small feature due to C zeta of Arg residues, primarily in the myelin basic proteins. The general similarity between the model lipid and intact myelin spectra suggested no major effects of protein on lipid mobility. We have also investigated human myelin samples as a function of developmental age (4, 15, 48 months and adult), and our results showed only small changes in overall lipid composition, although there were significant decreases in lipid hydrocarbon chain unsaturation with age, as determined by computer line-shape simulations of myelin and model compounds. The spectrum of an infant leukoencephalopathy myelin showed marked decreases in galactocerebrosides. Overall, the ability to resolve and assign over 40 resonances in the 13C MAS NMR spectra of myelin, and to detect changes as a function of development and disease, should provide a useful starting point for further more detailed studies of myelin membrane molecular motions, and function.